Inert lamp reflector



June 13, 1967 BlRD ET AL 3,325,666

INERT LAMP REFLECTOR Filed July 24, 1964 34 3O 24 40 44 FIG. I

52 22 6O 72 TO A. I2 82 D so FIG. 3

FIG. 5

INVENTORS ATTORNEY United States Patent 3,325,666 INERT LAMP REFLECTORGeorge R. Bird, Concord, and Willis E. Gray, Jr., Boston, Mass,assignors to Polaroid Corporation, Cambridge, Mass., a corporation ofDelaware Filed July 24, 1964, Ser. No. 384,984 7 Claims. (Cl. 313-113)This invention relates to electric incandescent lamps and moreparticularly to lamps adapted or designed to project carefullycontrolled or substantially collimated light beams such, for example, asslide or moving-picture projection lamps, photoflood lamps, and thelike.

A principal object of the present invention is to provide a new andimproved incandescent lamp adapted to project a beam of light in apredetermined direction and comprising an enclosure or bulb having anelectrical energy-translation element or filament sealed therein and acurved reflector so positioned within the bulb as to reflect lightemitted by the filament.

Another object of the invention is to provide an incandescent lamp ofthe above type in which the filament comprises tantalum carbide and theenvelope of the lamp includes a carbon-containing atmosphere.

Still another object of the invention is to provide a dichroic reflectorwhich will resist heat and the reactive gases of the lamp atmosphere.

A still further object of the invention is to provide an incandescentlamp of the character described in which the reflector comprises aplurality of layers of alternately highand low-refractive-indexsubstances, the high-refractive-index substance comprising a refractorymetal oxide selected from the group consisting of tantalum pentoxide,thorium dioxide, zirconium dioxide and hafnium dioxide.

()ther objects of the invention will, in part, be obvious and will inpart appear hereinafter.

The invention accordingly comprises the products possessing thefeatures, properties and the relation of elements which are exemplifiedin the following detailed disclosure, and the scope of the applicationof which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawing wherein:

FIGURE 1 is a front view of one embodiment of a lamp according to theinvention;

FIG. 2 is a side view, with parts cut away, of the lamp of FIGURE 1;

FIG. 3 is a rear view of the lamp;

FIG. 4 is a sectional view of the lamp of FIGURE 1, taken along thelines 4-4; and

FIG. 5 is an exaggerated, diagrammatic fragmentary sectional view of thereflector of the present invention.

Electric incandescent lamps provided with an internal reflector which isspaced or separated from the lamp bulb or envelope are well known. Lampsof this type which are adapted for providing or projecting a highconcentration of light energy are described, for example, in US. Patents2,979,634 and 2,980,818. In these patents, there is disclosed aprojection lamp or the like which comprises a light-transmitting bulbhaving supported therein a filament and a reflector of the desiredcurvature which is separate from the bulb. The filament and thereflector are aligned or positioned with respect to one another so as to3,325,666 Patented June 13, 1967 be very close and in reflectingrelationship. In order to give good life, the lamps are preferablyfilled with an inert gas such as nitrogen to reduce evaporation from thefilament which is usually a coiled tungsten wire.

It is very desirable that lamps of the above type provide as high abrightness and efliciency as is possible. The brightness and efiiciencyof lamps utilizing a tungsten incandescent body or filament is nogreater than that ohtainable at the melting point of tungsten which isabout 3380 C. Brightnesses and efiiciencies higher than that obtainablewith tungsten can be obtained with lamps of the above type by utilizinga filament comprising tantalum carbide (TaC). However, when such acarbide filament is utilized, it is necessary to provide a lampatmosphere comprising sufficient carbon to prevent the carbide filamentfrom deteriorating or decomposing at operating temperatures into anundesirable subcarbide or free metal and carbon. This lamp atmosphere,which can be termed a carbide-maintaining atmosphere, can be provided bya number of materials or sources. For example, satisfactory atmospherescan be provided by initially introducing into the bulb a gaseous mixturecomprising a hydrocarbon and hydrogen or a hydrocarbon, hydrogen andnitrogen or a halogenated hydrocarbon and hydrogen or the like.Incandescent lamps having a filament comprising tantalum carbidepositioned within a carbide-maintaining atmosphere are more fullydescribed, for example, in US. Patents 2,596,469, 3,022,438 and3,022,439.

It has been found that while projection lamps or the like provided witha heretofore suggested reflector and a filament comprising tantalumcarbide produced very desirable brightness and efficiencies, they hadrelatively short useful lives due to the rapid and severe deteriorationof the reflector. This deterioration of heretofore proposed internallamp reflectors is believed to be due to the very high operatingtemperatures to which the reflector is subjected and to the attack oraction upon the reflector, at elevated temperatures, of reactive gasessuch, for example, as reducing gases, halogens or the like, which arepresent in the carbide-maintaining atmosphere.

In the present invention, an incandescent lamp is provided whichcontains an internal reflector that is highly resistant to heat and toattack by reactive gases which may be present in the lamp bulb. In oneembodiment of the invention, there is provided an incandescent lampwhich comprises a bulb having a light-transmitting portion, the bulbhaving supported or mounted therewithin, in reflecting relationship, afilament and a curved internal reflector which comprises a plurality oflayers of alternately highand low-refractive-index materials, thehighrefractive-index material comprising a refractory metal oxideselected from the group consisting of tantalum pentoxide, thoriumdioxide, zirnconium dioxide and hafnium dioxide. Thelow-refractive-index material can be any one of many commonly used inthe production of interference filters, for example, an alkali metal oralkaline earth metal fluoride. Thus, one preferred reflector comprises areflecting surface or coating composed of an odd number of layers ofalternately high and low indices of refraction deposited or laid downupon the concave surface side of a suitable support such as, forexample, glass, the highrefractive-index layer comprising tantalumpentoxide and the low-refractive-index layer comprising a metal fluoridesuch as magnesium fluoride.

In one preferred embodiment of the invention, an in- :andescent lamp ofthe above type is provided wherein :he bulb includes a filamentcomprising tantalum carbide and a carbon-containing atmosphere formaintaining the desired carbide filament structure at operatingtemperatures.

While the drawings illustrate a specific lamp or structure, that is, aprojection lamp for, say, a slide or motionpicture projector, and theinvention will be described in connection with this lamp, it should beunderstood that the invention may, advantageously, be employed inconjunction with incandescent lamps generally, for example, photofloodlamps or other related structures adapted to project a highconcentration of light energy.

Referring now to the figures, a tubular bulb or envclope having alight-transmitting portion encloses a reflector 12 and a filament 14which is positioned at or near the focus of the reflector 12. The bulbwhich is generally of a high-transmissive materal such as glass issealed at the neck portion 16 thereof to a disc or wafer-like glassheader 18. As illustrated, the neck portion 16 of the bulb is of asmaller diameter than the main portion of the bulb. The other end of thebulb is rounded, as shown, and can be covered by a coating 20, which canbe of a colored or black ceramic or enamel glaze suitable for sealing tothe bulb. The use of this coating increases the radiation of heat fromthe top of the bulb so as to prevent overheating of the bulb.

In order to provide supports and external contacts, electricallyconducting lead-in wires or pins 22, 24, 26 and 28 of an appropriatemetal are sealed through individual nubs 30, 32, 34 and 36 of the glassheader 18.

A metal cap 38 is attached to the neck 16 of the bulb 10 by a suitablecement, and carries an outwardly projecting piece 40, which covers andprotects the sealed exhaust tube 42, and, together with the keyway 44,also serves to center the lamp in its socket. The glass nubs 30, 32, 34and 36 pass through holes in the bottom of the cap 38 so that thelead-in contact wires 22, 24, 26 and 28 are insulated from the cap.

The reflector 12 comprises a curved support or base member 46 having aconvex and a concave side. A lightreflecting coating 48 is provided onthe concave side of the reflector 12. The nature of the light-reflectingcoating or surface will be discussed in detail hereinafter. The convexback surface of the support 46 which can, for example, be of glass, isprovided with nubs 50, 52, 54 and 56. A short wire 58, 60, 62 and 64extends out of each nub, and is sealed therein. Support wires 66, 68, 70and 72 are secured to the aforementioned short wires, one of the latterwires to each support wire. The longer support wires 66 and 68 extenddownwardly and toward each other, being curved slightly to conform tothe curvature of the reflector support 46, and are attached at theirbottom ends to a metal cross-wire 74, which extends between the leadinwires 26 and 28, being secured to each of them. Wires 60 and 64,extending from nubs 52 and 56, respectively, are affixed to supportwires 70 and 72. The support wires 70 and 72 curve downwardly around theconvex back of the support 46 and are joined to lead-in wires 22 and 24,respectively.

In order to support and make electrical connections with the filament14, electrically conductive metal support wires 76 and 78 are secured,e.g., welded, to the ends thereof, one wire to each end. The filamentsupport wires 76 and 78 are curved to form bights 80 and 82 at theirbottom ends and are attached near these ends to the sup port wires 70and 72, respectively, which, as above stated, are attached to thelead-in wires 22 and 24.

In addition to the above-described means for firmly supporting thereflector 12 and the filament 14 within the bulb 10 and in the desiredoptical relationship or alignment with each other, other well knownsupport means may be employed. For example, the reflector can besupported inside the bulb and held firmly in the proper reflectingrelationship to the filament in the manner described in U.S. Patent2,980,818.

As shown in the figures, the elongated coiled filament 14, to which thereflector 12 is in correct reflecting relationship, is supported withinthe bulb so that its axis is transversely disposed to the longitudinalaxis of the bulb. The concave reflector 12, supported in spaced relationinside the bulb 10, is shown as having its axis transverse to that ofthe filament and to the longitudinal axis of the bulb, so as to reflector direct light from the filament 14 in the direction of maximum lightemission of the filament outwardly through the tubular walls of thebulb. Substantially elliptical or spherical reflectors can be utilized,the positioning of a filament with respect to such reflectors to obtaindesired collimated light beams being well known in the art.

In order to get as large a useful reflecting surface as possible into atubular bulb, or into a bulb having a circular opening through which thereflector is to be inserted, the perimeter of the reflector 12 is madenon-circular, that is, the reflector has its sides cut off to a slightlycurved configuration as shown in the figures. The reflector can,appropriately, be cut off at each side in a plane at about 15 to theplane of the front of the reflector. The specific reflectorconfiguration shown in the drawing is more fully described in U.S.Patents 2,980,818 and 2,994,- 799.

The filament can be of any desired configuration as, for example,straight, coiled, crimped, coiled-coil or otherwise shaped. Preferably,however, either an elongated coiled filament or a coiled-coil filamentis employed in the lamps described. The filament size is preferably suchthat it is small compared with the reflect-0r.

The filament may comprise an electrically-conductive refractory materialsuch as tungsten, in which instance the lamp is usually provided with asuitable pressure of of an inert gas such as nitrogen, argon, or thelike. Preferably, however, the electrically incandescent body orfilament comprises tantalum carbide since it provides a higherbrightness than conventional tungsten filaments. When a filamentcomprising tantalum carbide is utilized in a lamp of the characterdescribed, the lamp must be provided with a suitable carbon-containingatmosphere in order to maintain the desired carbide filament structureat operating temperatures. If the lamp is devoid of or containsinsuflicient carbon in the atmosphere at operating temperatures,decarburization of the filament takes place and results in the rapiddeterioration and failure of the filament.

Heretofore, it has been proposed to employ an atmosphere the elements ofwhich interact with each other and with the carbide filament so as tosubstantially retard or prevent decarburization of the tantalum carbidefilament. The preferred carbide-maintaining atmosphere can be providedby initially introducing into the lamp a suitable pressure of a gaseousmixture comprising hydrogen and hydrocarbon such as acetylene, ethylene,benzene, etc., or a gaseous mixture comprising hydrogen and a halogenated hydrocarbon such as carbon tetrachloride, trichloroiodomethane,etc., or a gaseous mixture comprising hydrogen, a hydrocarbon andnitrogen. An inert gas of low heat conductivity such, for example, asargon can also be included as a gaseous lamp filling. Incandescent lampscomprising a tantalum carbide filament positioned Within acarbide-maintaining atmosphere are known in the art as shown, forexample, in U.S. Patents 2,596,469, 3,022,438 and 3,022,439.Accordingly, the teachings thereof, particularly with respect to thecarbide-maintaining atmospheres which can be utilized, are incorporatedby reference herein.

The use of a filament comprising tantalum carbide in projection lamps,or the like, is particularly desirable since its very high melting pointpermits the lamp to be operated at temperatures which are well above themelting point of tungsten. The higher temperatures produce a higher orgreater brightness than that obtainable with conventional filaments suchas tungsten. It has been found, however, that heretofore-proposedinternal lamp reflectors are severely attacked and quickly rendereduseless when subjected to the higher operating temperatures associatedwith carbide maintaining atmospheres. The severe and rapid deteriorationof the reflector can be attributed to the very high temperaturesgenerated by the closely located filament and to the action at theaforesaid temperatures of reactive gases, e.g. reducing gases, halogensor the like, which are present in the carbide-maintaining atmosphere.

In the present invention, an internal lamp reflector is provided whichis highly resistant to heat and to attack by reactive gases,particularly reducing gases and halogens which are present in thecarbide-maintaining atmosphere. The present reflector may,appropriately, be termed a dichroic reflector, in the sense that itreflects light rays of one wavelength and transmits light rays ofanother wavelength, or a multilayer dielectric reflector, in the sensethat its layers are individually optically transparent but togetherreflecting. The reflector comprises a curved support 46, which can be ofglass, quartz, or the like, having a multilayer light-reflecting coatingor surface 48 on the concave side thereof. The light-reflecting coating48 comprises a plurality of layers 90 of a high-refractive-indexmaterial or substance alternated with a plurality of layers 92 of alow-refractive-index material or substance. The high-refractive-indexmaterial is a white refractory metal oxide and more particularly arefractory metal oxide selected from the group consisting of tantalumpentoxide (Ta O thorium dioxide (ThO zirconium dioxide (ZrO and hafniumdioxide (HfO The low-refractiveindex material can be any one of the manycommonly used in the production of reflection-reducing coatings anddichroic filters, for example, a metal fluoride. In one embodiment ofthe invention the low-refractive-index material is selected from thegroup consisting of the alkali metal and alkaline earth metal fluorides.Among the suit able metal fluorides which can be used, mention may bemade of lithium fluoride (LiF), sodium fluoride (NaF), calcium fluoride(CaF magnesium fluoride (MgF and cryolite (Na AlF The dichroic reflectorof the present invention can be formed by utilizing well-known vacuumcoating techniques for the deposition of thin films or layers ofdielectric materials. Through variation of the multilayer coating, thetransmission and deflectivity characteristics of the reflector can bevaried. The thickness, number, sequence and composition of theindividual layers are carefully controlled so as to provide a reflectorwhich will substantially reflect most of the light rays from thefilament which are incident thereupon and yet pass or transmit a largepart of the heat radiation. In other words, there is provided areflector having a reflectivity in the visible region which issubstantially as high as possible and a transmission of the radiationabove about 7000 A., that is, in the infrared which, also, issubstantially as high as possible.

The reflector preferably comprises a coating composed of an odd numberof thin optically-dielectric layers. In one embodiment both the layerlocated adjacent to the support 46 and the outermost layer of thereflecting coating, that is, the layer closest to the filament, compriseor are principally composed of a high-refractive-index material. Thereflector can comprise, for example, three, five, seven, nine, etc.layers, the greater number of which are of high refractive index, e.g.,comprising a refractory metal oxide, heretofore noted.

While the thicknesses of the layers can be varied, they are, preferably,of a thickness of the order of a quarter wavelength. Moreover, while allof the high-refractive-index layers preferably comprise a given materialand all of the low-refractive-index layers preferably comprise annothergiven material, it should be mentioned that more 6 than one material canbe utilized in the makeup of th highor low-index-layers of thereflector. The thickness number, sequence and composition of the layersare readi 1y determinable by one skilled in the art, once the reflecting and transmitting characteristics desired in the reflecto are decidedupon.

In one preferred embodiment of the invention the re flector includes aplurality of layers of alternately high and low-refractive-indexmaterials, the high-refractive-in dex dielectric material comprisingtantalum pentoxide am the low-refractive-index dielectric materialcomprising metal fluoride such, for example, as magnesium fluoride ocryolite. It has been found that such a reflector is no appreciablyaffected by high operating temperatures or b reactive gases present inthe lamp atmosphere.

Attempted deposition of layers of the preferred tanta lum pentoxide byevaporating it in a vacuum and permit ting the vapors produced therebyto condense upon th reflector have often resulted instead in theformation 0 undesirably colored layers comprising lower tantalu'noxides. It has been found, however, that the desired es sentiallycolorless tantalum pentoxide layers can b achieved by several methods.For example, after the dc; osition of an undesirably colored,oxygen-deficient tanta lum oxide layer, such a layer can be converted totanta lum pentoxide by suitably heating it in an atmosphere con tainingoxygen, that is, in air. The production of tantalur pentoxide layers canalso be assured by heating the tanta lum oxide deposit during theformation thereof. A suitabl pressure of an oxygen-containing gas isintroduced int the system during such coating and heating in order tfacilitate the formation of a tantalum pentoxide deposit It is alsopossible to first deposit all the layers and the heat the compositestructure in an oxygen-containing a1 mosphere to convert any lowertantalum oxides preset therein to the desired tantalum pentoxide form.The abov methods can also be utilized in connection with the forma tionof layers of the other preferred refractory met: oxides, abovementioned.

Since certain changes may be made in the above prod ucts withoutdeparting from the scope of the inventio herein involved, it is intendedthat all matter containe in the above description or shown in theaccompanyin drawing shall be interpreted as illustrative and not inlimiting sense.

What is claimed is:

1. An incandescent lamp comprising a filament whic comprises tantalumcarbide and is adapted to operate 2 a very high temperature, a concavemultilayer reflector i reflecting positional relationship to saidfilament, a seale bulb having a light-transmitting portion enclosing saifilament and said reflector, and a carbon-containing at mosphere alsocontained within said bulb and adapted t maintain the tantalum carbidestructure of said filamer at said high operating temperature, saidreflector bein adapted to reflect light in a direction forwardly througsaid light-transmitting bulb portion and transmit infrare radiation in agenerally reverse direction and comprisin a curved support, a pluralityof layers of alternately higl and low-refractive-index substances whichare of type highly resistant both to heat radiating from said filamerand to attack by any reactive gases present in said atmos phere, saidhigh-refra-ctive-index substance comprising refractory metal oxideselected from the group consistin of tantalum pentoxide, thoriumdioxide, zirconium d oxide and hafnium dioxide.

2. An incandescent lamp according to claim 1 wherei thelow-refractive-index substance is selected from th group consisting ofthe alkali metal and alkaline eart metal fluorides.

3. An incandescent lamp according to claim 1 wherei saidcarbon-containing atmosphere comprises a hydroca1 bon and hydrogen.

4. An incandescent lamp according to claim 1 wherei saidcarbon-containing atmosphere comprises a hydrocar bon, hydrogen andnitrogen.

References Cited UNITED STATES PATENTS Geffcken et :11. 1l7--124 Koch88-24 Cooper 313-222 Scoledge et al. 313-113 Thelen 881 JAMES WLAWRENCE, Primary Examiner.

P. C. DEMEO, Assistant Examiner.

1. AN INCADESCENT LAMP COMPRISING A FILAMENT WHICH COMPRISES TANTALUMCARBIDE AND IS ADAPTED TO OPERATE AT A VERY HIGH TEMPERATURE, A CONCAVEMULTILAYER REFLECTOR IN REFLECTING POSITONAL RELATIONSHIP TO SAIDFILAMENT, A SEALED BULB HAVING A LIGHT-TRANSMITTING PORTION ENCLOSINGSAID FILAMENT AND SAID REFLECTOR, AND A CARBON-CONTAINING ATMOSPHEREALSO CONTAINED WITHIN SAID A CARBON-CONTAINING ATMAINTAIN THE TANTALUMCARBIDE STRUCTURE OF SAID FILAMENT AT SAID HIGH OPERATING TEMPERATURE,SAID REFLECTOR BEING ADAPTED TO REFLECT LIGHT IN A DIRECTION FORWARDLYTHROUGH SAID LIGHT-TRANSMITTING BULB PORTION AND TRANSMIT INFRAREDRADIATION IN A GENERALLY REVERSE DIRECTION AND COMPRISING A CURVEDSUPPORT, A PLURALITY OF LAYERS OF ALTERNATELY HIGHANDLOW-REFRACTIVE-INDEX SUBSTANCES WHICH ARE OF TYPES HIGHLY RESISTANT BOTHTO HEAT RADIATING FROM SAID FILAMENT AND TO ATTACK BY ANY REACTIVE GASESPRESENT IN SAID ATMOSPHERE, SAID HIGH-REFRACTIVE-INDEX SUBSTANCESCOMPRISING A REFRACTORY METAL OXIDE SELECTED FROM THE GROUP CONSISTINGOF TANTALUM PENTOXIDE, THORIUM DIOXIDE, ZIRCONIUM DIOXIDE AND HAFNIUMDIOXIDE.